On the vertex of the dorsal skull the premaxillary bone extends forward of the nasal and frontal bones. Separates from Berardius and Ziphius.

A sulcus (groove) running along the middle of the combined surfaces of the nasal bones so depresses their combined middle that it is the lateral portion of each nasal bone that reaches farthest forward on the vertex. Separates from Tasmacetus and Indopacetus.

When the skull is upright and the long axis of the anterior half of the beak is horizontal, a horizontal plane transecting the summit of either maxillary prominence transects the mesethmoid bone. Separates from Hyperoodon.

Right premaxilla extends posteriorly beyond the right nasal a distance exceeding 70% of dorsal length of right nasal. Separates from M. ginkgodens.

Antorbital notches form obtuse angles. Separates from M. peruvianus.

A transverse plane perpendicular to the long axis of the rostrum and transecting the apex of the left antorbital notch, very nearly also transects the anterior most point reached by the pterygoid sinus. Separates from M. densirostris.